Why is [tex] \pi[/tex] in the numerator in the last term?
The mislabelled "r" is just a mistake, let this be a lesson to keep an eye on what you're
doing from now on :tongue: but please tell me why [tex] \pi[/tex] is in the numerator?

i thought it was because original when it is brought up and r is put to the power of a negative it becomes a negative number...? probably wrong what is the right way to do it i always get mixed up with the order of how the terms and numbers go :(

Everything except the [tex] \pi[/tex] is correct, you need to understand why what
you've written is incorrect.

If you just cross multiply the right hand side of the last latex message I wrote & then get it looking pretty, you're almost there. All you have to do then is divide both sides by h
& then take a limit & you'll have an answer.

If you look at the equation you'll see that I multiplied both of the crazy fractions by terms that are, in reality, only equal to 1. Doing this is smart because you're allowed to multiply a number by 1, even if it's written as [tex] \frac{2}{2} [/tex] or even [tex] \frac{(r + h)^2}{(r + h)^2} [/tex]

It's the same thing as "cross multiplying" except this way makes sense as opposed to some magical trick :tongue:

See!!! On the left hand side you have the definition of the derivative clearly shown!
On the right you need to send h to 0 & look, a term dissappears & the h in the denominator
also dissappears so that you get a clean answer.

you sir are a legend and yes actually funnily enough my modern history teacher recommended this site to me when i had spares today, she said it was epic with calculus etc so obviously she wasn't bsing lol xD thanks man i fully see it now (btw i did have it written down in lined book while i was doing it i just really couldn't remember xD)